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Searching for Intelligence in Our Genes
IQ is easy to measure and reflects something real. But
scientists hunting among our genes for the factors that
shape intelligence are discovering they are more elusive
than expected
By Carl Zimmer
In Robert Plomin’s line of work, patience is essential. Plomin, a behavioral geneticist at
the Institute of Psychiatry in London, wants to understand the nature of intelligence. As
part of his research, he has been watching thousands of children grow up. Plomin asks the
children questions such as “What do water and milk have in common?” and “In what
direction does the sun set?” At first he and his colleagues quizzed the children in person
or over the telephone. Today many of those children are in their early teens, and they take
their tests on the Internet.
In one sense, the research has been a rousing success. The children who take the tests are
all twins, and throughout the study identical twins have tended to get scores closer to
each other than those of nonidentical twins, who in turn have closer scores than unrelated
children. These results—along with similar ones from other studies—make clear to the
scientists that genes have an important influence on how children score on intelligence
tests.
But Plomin wants to know more. He wants to find the specific genes that are doing the
influencing. And now he has a tool for pinpointing genes that he could not have even
dreamed of when he began quizzing children. Plomin and his colleagues have been
scanning the genes of his subjects with a device called a microarray, a small chip that can
recognize half a million distinctive snippets of DNA. The combination of this powerful
tool with a huge number of children to study meant that he could detect genes that had
only a tiny effect on the variation in scores.
Still, when Plomin and his co-workers unveiled the results of their microarray study—the
biggest dragnet for intelligence-linked genes ever undertaken—they were
underwhelming. The researchers found only six genetic markers that showed any sign of
having an influence on the test scores. When they ran stringent statistical tests to see if
the results were flukes, only one gene passed. It accounted for 0.4 percent of the variation
in the scores. And to cap it all off, no one knows what the gene does in the body.
“It’s a real drag in some ways,” Plomin says.
Plomin’s experience is a typical one for scientists who study intelligence. Along with
using microarrays, they are employing brain scans and other sophisticated technologies to
document some of the intricate dance steps that genes and environment take together in
the development of intelligence. They are beginning to see how differences in
intelligence are reflected in the structure and function of the brain. Some scientists have
even begun to build a new vision of intelligence as a reflection of the ways in which
information flows through the brain. But for all these advances, intelligence remains a
profound mystery. “It’s amazing the extent to which we know very little,” says Wendy
Johnson, a psychologist at the University of Minnesota.
Hidden in Plain Sight
In some ways, intelligence is very simple. “It’s something that everybody observes in
others,” says Eric Turkheimer of the University of Virginia. “Everybody knows that some
people are smarter than others, whatever it means technically. It’s something you sense in
people when you talk to them.”
Yet that kind of gut instinct does not translate easily into a scientific definition. In 1996
the American Psychological Association issued a report on intelligence, which stated
only that “individuals differ from one another in their ability to understand complex
ideas, to adapt effectively to the environment, to learn from experience, to engage in
various forms of reasoning, to overcome obstacles by taking thought.”
To measure these differences, psychologists in the early 1900s invented tests of various
kinds of thought, such as math, spatial reasoning and verbal skills. To compare scores on
one type of test to those on another, some psychologists developed standard scales of
intelligence. The most familiar of them is the intelligence quotient, which is produced by
setting the average score at 100.
IQ scores are not arbitrary numbers, however. Psychologists can use them to make strong
predictions about other features of people’s lives. It is possible to make reasonably good
predictions, based on IQ scores in childhood, about how well people will fare in school
and in the workplace. People with high IQs even tend to live longer than average.
“If you have an IQ score, does that tell you everything about a person’s cognitive
strengths and weaknesses? No,” says Richard J. Haier of the University of California,
Irvine. But even a simple number has the potential to say a lot about a person. “When you
go see your doctor, what’s the first thing that happens? Somebody takes your blood
pressure and temperature. So you get two numbers. No one would say blood pressure and
temperature summarize everything about your health, but they are key numbers.”
Then what underlies an intelligence score? “It’s certainly tapping something,” says Philip
Shaw, a psychiatrist at the National Institute of Mental Health (NIMH). The most
influential theory of what the score reflects is more than a century old. In 1904
psychologist Charles Spearman observed that people who did well on one kind of test
tended to do well on others. The link from one score to another was not very tight, but
Spearman saw enough of a connection to declare that it was the result of something he
called a g factor, short for general intelligence factor.
How general intelligence arose from the brain, Spearman could not say. In recent
decades, scientists have searched for an answer by finding patterns in the test scores of
large groups of people. Roughly speaking, there are two possible sources for these
variations. Environmental influences—anything from the way children are raised by their
parents to the diseases they may suffer as they develop—are one source. Genes are
another. Genes may shape the brain in ways that make individuals better or worse at
answering questions on intelligence tests.
Starting in the 1960s, scientists have gotten clues about the roles of genes and
environment by studying twins. To see why twins are so important to intelligence
researchers, imagine that a pair of identical twins are separated as babies and adopted by
different parents. They have the same genes but experience different environments. If
their genes have no influence at all on their intelligence test scores, then you would
expect that the scores would be no more similar to each other than those of two unrelated
people. Yet if genes do play a critical part in intelligence, identical twins should be more
similar.
“Two people with the same genes correlate as much as a person does with himself a year
later,” Plomin says. “Identical twins reared apart are almost as similar as identical twins
reared together.” But these similarities also take time to emerge. “By the age of 16 these
adopted-away children resemble their biological parents’ IQ just as much as kids do who
are reared by their biological parents,” Plomin adds.
Results such as these persuaded Plomin that genes have a crucial role in intelligence,
although they clearly do not act alone. “That’s what led me to say, ‘What we need to do
is begin to find some genes,’” he says.
Uncharted Territory
In the early 1990s, when Plomin started his search for genes, he had little company. “I
knew nobody else would be crazy enough to do it,” he remarks.
Plomin could not simply scan the human genome, because it had not been mapped yet.
But geneticists had identified a number of genes that, if mutated in certain ways, were
associated with mental retardation. Other variations in those genes, Plomin reasoned,
might produce subtler differences in intelligence. He and his colleagues compared
children who scored well or poorly on intelligence tests. They looked for variants of the
100 genes that showed up unusually often in one group or the other. “We didn’t really
fiSo Plomin expanded the search. Rather than looking at a predefined set of genes, he
mapped thousands of genetic markers sprinkled across the chromosomes of his subjects.
If a marker turned up frequently in high- or low-scoring students, there might be an
intelligence-linked gene not far away. He and his colleagues added more children to their
study so that they could detect genes with weaker effects. At one point in the research,
Plomin thought he had found an authentic link between intelligence and a gene known as
IGF2R that encodes a growth factor receptor which is active in the brain. But when he
and others tried to replicate the result, they failed. “It doesn’t look like that has panned
out,” he says.
Plomin suspected that he needed more genetic markers to find intelligence genes. When
eggs and sperm develop, their chromosomes swap segments of DNA. The closer two
segments of DNA are to each other, the more likely they are to be passed down together.
But in Plomin’s early studies, millions of DNA nucleotides separated each pair of
markers. It was possible that intelligence genes were so far from a genetic marker that
they were sometimes getting passed down together and sometimes not. He needed a
much denser set of genetic markers to reduce the chance of this happening.
It was with great delight that Plomin got his hands on microarrays that could detect
500,000 genetic markers—hundreds of times more than he had previously used. He and
his colleagues got cheek swabs from 7,000 children, isolated their DNA, and ran it
through the microarrays. And once more the results were disappointing.
“I’m not willing to say that we have found genes for intelligence,” Plomin declares,
“because there have been so many false positives. They’re such small effects that you’re
going to have to replicate them in many studies to feel very confident about them.”
Failing to find genes for intelligence has, in itself, been very instructive for Plomin. Twin
studies continue to persuade him that the genes exist. “There is ultimately DNA variation
responsible for it,” he says. But each of the variations detected so far only makes a tiny
contribution to differences in intelligence. “I think nobody thought that the biggest effects
would account for less than 1 percent,” Plomin points out.
That means that there must be hundreds—perhaps thousands—of genes that together
produce the full range of gene-based variation in intelligence. Plomin doubts that some
genes are specialized just for verbal skills and that other genes are just for spatial
understanding. In twin studies, individuals tend to have similar scores on tests for all of
those different kinds of intelligence. If genes belonged to specialized sets, a person could
inherit one kind of aptitude and not the others.
Plomin also surmises that his results offer some hints as to how genes influence
intelligence in the brain. “If there are many genes of small effect, it’s highly unlikely that
they are all going to focus in one area of the brain,” he argues. Instead the genes may be
influencing a large network of brain regions. And each of those intelligence-associated
genes may produce many different effects in different parts of the brain.
The ultimate test of Plomin’s hypothesis will have to wait until scientists finally put
together a list of genes that have an indisputable effect on how the brain works and that
are associated with intelligence scores. That list may take a long time to come together,
but Plomin is encouraged by new results from an entirely different line of research: a
burst of new neuroimaging studies that attempt to find the mark of intelligence in the
brain itself.
The Shape of Intelligence
Shaw and his colleagues at the NIMH have been analyzing brain scans of schoolchildren.
Researchers have made images of their developing brains once a year, and Shaw has
focused much of his attention on what the pictures reveal about the growth of the cortex,
the outer rind of the brain where the most sophisticated information processing takes
place. The cortex continues to change shape and structure until people reach their early
20s. And Shaw has found that differences in intelligence test scores are reflected in how
brains develop.
In all children the cortex gets thicker as new neurons grow and produce new branches.
Then the cortex thins out as branches are pruned. But in some parts of the cortex, Shaw
found, development took a different course in children with different levels of
intelligence. “The superclever kids started off very thin,” Shaw says. “They got really
relatively thicker, but in adolescence they got thinner again very quickly.”
A similar pattern has emerged from studies on adult brains. Researchers have found that
people with high intelligence scores tend to have certain regions of the cortex that are
larger than average. Shaw expects that some of those patterns will turn out to be the result
of the environment. But these regions of the cortex tend to be the same size in twins,
indicating that genes are responsible for some of the difference as well.
In recent years, scientists have also published a number of studies in which they claim to
have found distinctive patterns of brain functioning in people who score high on
intelligence tests. Recently Haier and Rex Eugene Jung of the University of New Mexico
surveyed 37 studies examining regional brain size or activity to look for an overall
pattern to their results. As Plomin would have predicted, Haier and Jung found no one
“intelligence spot” in the brain. Instead they identified a number of significant regions
scattered around the cortex. Other studies have implicated each of these regions in
different kinds of cognition. “It looks like intelligence is built on these fundamental
cognitive processes, like attention and memory, and maybe language ability,” Haier says.
Along with describing the gray matter tissue that makes up the cortex, these studies also
find the signature of intelligence in the white matter that links distant parts of the cortex
to one another. People with high intelligence tend to have tracts of white matter that are
more organized than other people. “The white matter is like the wiring,” Haier says. “If
you think about it, you know, intelligence really requires processing power and speed; the
white matter would give it the speed; the gray matter would give it the processing
power.”
Haier suggests that these parts of the “intelligence network” may work differently in
different people. “You can think about being very intelligent because you have a lot of
speed and a lot of processing—you have both,” he says. “Or you can think about a lot of
one and less of the other. All these combinations may produce the same ultimate result,
so you may have two equally intelligent people, but their brains are fundamentally
arriving at that behavior, however you’re measuring it, in different ways.”
Haier acknowledges that these ideas are little more than speculation. Still, he argues that
neuroimaging has already given scientists a far more solid understanding of intelligence.
“I can predict full-scale IQ with the amount of gray matter in a small number of areas,”
he says. Haier suspects that in the near future, 10 minutes in a magnetic resonance
imaging scanner may reveal as much about high school students as four hours taking an
SAT exam.
Some psychologists are not quite ready to take that step. They do not think IQ and g
should be endowed with a deeper significance than they deserve. For one thing, there is
much beyond the life of the mind than mentally rotating cubes and completing analogies.
“I think human intelligence is multifaceted and very complex,” the University of
Virginia’s Turkheimer says. Unfortunately, he adds, barely any work has been done on
other facets of intelligence.
“We can use g for a lot of useful things, but I don’t believe it follows from that that
human intelligence is a unitary thing called g that we can find in a literal way in the
brain,” he says. Longitude and latitude are useful, too, for navigation, he notes, but that
does not mean there is actually a grid carved into the earth.
Johnson of the University of Minnesota defends the g factor as tapping into something
important in the brain, but, like Haier, she does not think it is a one-size-fits-all general
intelligence. “While there is something general about intelligence, what makes my
intelligence general is not the same thing that makes your intelligence general or any
other person’s,” she says. “Our brains are plastic enough that we put together, each of us,
a different kind of general intelligence.”
Pinpointing the role genes have in producing different kinds of intelligence will no doubt
be very difficult. And it is entirely possible that a list of intelligence-linked genes may
include many that do not actually have brain-specific functions. Turkheimer offers the
following thought experiment: Imagine a gene that is related to the width of a woman’s
birth canal. Women who carry a gene for a narrow birth canal tend to have more trouble
in labor, and their babies run a higher risk of being oxygen-deprived. As a result, their
babies, on average, have IQ scores a couple points below those who have a different
version of the gene. And some of those children will also carry the narrow-canal gene.
“These babies are going to have a gene that’s correlated with low IQ,” Turkheimer says.
“So do you conclude that that’s an IQ gene? Well, not really; it’s a birth-canal gene. The
ways that genes could correlate with IQ are so variable that it’s almost impossible to
know.”
Turkheimer’s own research illustrates another kind of complexity in the link from genes
to intelligence: genes do not act in isolation from the environment. In fact, the same gene
can have different effects in different environments. Turkheimer was led to this
realization when he noticed that the large twin studies on intelligence contained few poor
children. “Very poor people don’t have the time or the resources or the interest to do
volunteer studies,” he says.
Other databases, Turkheimer discovered, have more poor children in their ranks. He was
able to analyze the test scores of hundreds of twins, taking into consideration their
socioeconomic status—a category based on factors that include a family’s income and the
education level of the parents. He found that the strength of genes’ effect depended on the
socioeconomic status of the children. In children from affluent families, about 60 percent
of the variance in IQ scores could be accounted for by genes. For children from
impoverished families, on the other hand, genes accounted for almost none.
Turkheimer and his colleagues published these results in 2003; in May 2007 they
replicated the pattern with another database. Instead of comparing IQ scores, the
researchers looked at how 839 pairs of twins fared on the National Merit Scholarship
Qualifying Test in 1962. Once more genes played little role in the variance of scores
among poor children and played a far stronger one in more affluent children. Turkheimer
posits that poverty brings with it powerful environmental forces that may shape
intelligence from the womb through school and onward. But when children grow up in
the relative stability of an affluent home, gene-based differences can begin to make
themselves felt.
As if this complexity was not enough, scientists are also finding that genes, in turn, can
alter the effect that the environment has on our intelligence. Last year British scientists
found an association between breast-feeding and a boost in IQ test scores—but only if
children carried a particular variant of a particular gene. If they carried another variant,
breast-fed children scored no differently than children who drank formula.
Genes may also influence behavior in ways that influence how intelligence develops.
“People create their own environments,” Johnson says. “If you see a kid who’s really
interested in art or math, you’re just more likely to go out and get him a math book or
some crayons. So they’ll practice it and become more different from the kid who doesn’t
have the math book. Parents respond to what the kid does. Our models don’t measure that
well at all.”
This effect might explain one of the most puzzling patterns in twin studies on
intelligence: how the influence of genes becomes stronger on test scores as people get
older. Genes may affect how people mold their intellectual environment. Choosing to
seek out new experiences, reading books and engaging in conversations may alter the
brain. And as children grow up and take over control of their own lives, this effect may
get stronger.
“Intelligence is kind of an emergent property of the brain,” Shaw says. “The idea that
you’re born with 15 genes, and they set in stone how intelligent you’re going to be and
how your brain is going to develop, is almost certainly wrong.”
Why Study Intelligence?
Intelligence may be enormously complex, and scientists may have made frustratingly
little progress in understanding it. Yet many experts on intelligence still see some
practical values in continuing the quest. Haier, for example, hopes that a brain-based
understanding of intelligence will help teachers design strategies for educating children
most effectively.
“It’s very important as we enter the 21st century to maximize and optimize education for
people,” he argues. “That’s what’s at stake.”
By understanding people’s genetic profiles, Plomin suggests, it may be possible to find
the best ways to foster learning. If, as he anticipates, microarray studies finally do reveal
intelligence genes, it may then be possible to test children for which versions they carry.
“You could get an index of genetic risk,” Plomin says. “You could see which kids are at
genetic risk for reading disability, and you could then intervene. The hope is that you
could predict and intervene with programs to prevent those problems rather than waiting
until they occur in school.”
And for some psychologists, it is enough that intelligence is such an intriguing part of
human nature. “Intelligence and intelligence test scores are in many ways the best
predictor in all of psychology,” Turkheimer says. “That’s what makes it fascinating. If
you know my SAT scores and you want to know how I’m going to perform at practically
anything, those SAT scores are far from a perfect predictor, but they’re far better than
knowing about my personality. Intelligence really works. There’s this palpable
psychological quality that allows you to make predictions about humans but gets very
slippery when you try to tie it down in concrete numerical ways. So it’s just a very
interesting scientific problem.”